ABSTRACT: Strains measured on the surfaces of rectangular prisms machined from low porosity, low permeability rock cores are used to determine that portion of the porosity comprised of small low aperture cracks. In these measurements, which are similar to the the differential strain analysis technique, the linear microcrack strains are the difference between the observed response of the strain gage and the estimated intrinsic material strain. These crack strains may be displayed in a tensor form and principal directions and magnitudes calculated, the trace of the crack strain tensor further yields the microcrack porosity of the rock. Two example experiments in a weakly foliated granitic gneiss and a sedimentary dolomite indicate that these crack principal crack strains are not necessarily aligned with the in situ horizontal and vertical principal stresses as might be anticipated if the crack damage resulted solely from stress relief mechanisms after coring.

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